The invention relates to a method and apparatus for drying ceramic articles using dielectric energy without distortions and other surface flaws.
It has been a long standing manufacturing problem to efficiently produce ceramic structures without both structural defects and skin or surface related flaws or "fissures," which together account for a majority of all rejects in the manufacturing process. When used as a catalytic support for an automotive exhaust system, fissures provide focal points for stress and heat differentials in the ceramic support during automotive exhaust gas cycling, and eventually spread and cause failure of the catalytic support material.
Surface defects are particularly problematic in continuous extrusion processes. In a typical manufacturing process, as the wet or green log or ceramic structure leaves the extruder, it is conveyed by air bearings to a ceramic carrier or setter which is generally contoured to the shape of the log. Honeycomb extrusions of very large (up to 13.5 inch diameter) diesel particulate filters, for example, are dried initially in a dielectric oven. Each "log" or continuous extrudate rests on a horizontal position on a ceramic tray or carrier and is conveyed through the oven on a conveyor belt. The log-bearing setters or carriers are carried into a dielectric oven where the logs are dried. The dried logs are then cut and fired to form the catalyst material supports. Most surface defects occur as the setter/logs travel to and through the dielectric drier. This is because the log leaving the extruder has a high water content and evaporation from the log begins immediately upon its exit from the extruder. Generally, the drying is not uniform. For instance, the bottom of the log is much dryer than the top of the log. It is theorized that the underside of the log dries faster than the rest of the material due to its proximity to the lower electrode in the dielectric oven. Also, both the leading and trailing surfaces of the log tend to lose water very slowly, often retaining all or close to all of its original water. As a result of this uneven drying, a stress differential is created between the top and bottom surfaces of the log causing fissures to form, especially on the top surface. The fissures form to alleviate the stress created between the quicker shrinking bottom surface layer and the top layer. Because the bottom of the log dries much faster, any additional energy absorbed by the log causes it to overheat and/or burn in the driest regions. Finally, the stresses are retained in the ware and may cause cracking during the firing or sintering step.
Uneven drying in a dielectric oven is caused by variations in the energy field. Generally, the field is strongest in the regions nearest the electrodes, that is, above and below the log, and weakest at the leading and trailing regions of the log. Thus, the farther from the electrodes, the weaker the field. Also, interference from adjacent logs may contribute to the weakness of the field at the leading and trailing regions of the logs.
In the wet or green state, ceramic ware is easily distorted. This is particularly true with honeycomb structures having an intricate matrix of thin to very thin cell walls which easily slump or distort when wet. To minimize slumping and distortions, in the dielectric oven, it is often necessary to support the wet or green ceramic ware on a contoured tray or setter until the ware is sufficiently dry to maintain its shape. The problems encountered in the initial drying of ceramic ware are different and in addition to those which are later encountered when such ware are fired or sintered. Many of the setters which have been disclosed for sintering ceramic ware have been ceramic materials which in some cases have been shaped to minimize the occurrence of cracking and other problems encountered when ceramic ware is fired.
While various methods have been suggested for making setters or trays for supporting ceramic ware during the sintering or firing process, very little has been disclosed for supporting such ware in a dielectric drying environment.
A common type of tray used to support ceramic ware in the dielectric drying process is made from light weight, filled ceramic material which is either cast or machined to fit the shape of the ceramic ware. However, such ceramic trays are fragile and friable. They are easily chipped or broken in handling, producing dust particles which are irritating to the skin and throat. In addition, because the trays are fragile, they are often set in a strong plastic such as Lexan.RTM. or onto a flat plate of a glass fiber reinforced plastic. Like the ceramic trays, the Lexan.RTM. is also easily damaged in handling sometimes resulting in catastrophic failures. For example, in the event of arcing or overheating in the dielectric dryer, the Lexan.RTM. will melt, creating a major clean-up problem. As a result of these problems, ceramic trays are not cost effective. More importantly, ceramic trays absorb energy in the radio frequency range, thereby reducing the energy available for drying the ceramic ware. Also, because ceramic trays absorb dielectric energy in the radio frequency range, much of the absorbed energy is transferred by conduction to the bottom portion of the logs causing this section of the log to dry faster than the rest of the log.
Therefore, there continues to be a need for apparatus and methods for drying wet or green ceramic ware in a dielectric dryer without slumping or distorting the ceramic ware. Accordingly, it is the object of the present invention to provide a cost effective apparatus and method for supporting green ceramic ware while avoiding or significantly reducing the aforementioned problems.